Previous work on this project has led to identification of several LD homozygous individuals whose lymphocytes act as typing cells in MLC experiments. With these LD typing cells it will be possible to recognize the LD specificities "R", "r" and "jr". The first of these (R) is known to be a typing cell for part of the broad specificity HLA-DW4 as defined in the 6th International Histocompatibility Workshop. Further studies will include (1) characterization of the other two determinants, (2) population studies in patients with rheumatoid arthritis and controls, (3) family studies, and (4) studies in various special groups of patients including patients with juvenile rheumatoid arthritis, Sjogren's Syndrome, etc. BIBLIOGRAPHIC REFERENCES: Stastny, P. Mixed lymphocyte cultures (MLC) in adult patients with rheumatoid arthritis, Arthr. Rheum. 18:428, 1975 (abstract). Stastny, P. and Fink, C.W. Mixed lymphocyte culture (MLC) and HL-A antigens in children with rheumatoid arthritis. Arthr. Rheum. 18:428, 1975 (abstract).